In telephony, line polarity reversal is used as a method of call status signaling in analog telephony interfaces. This event is used primarily to frame the beginning and ending of a telephone communication transaction, in order to prevent call collisions, known as a glare condition, in automated telephony equipment. Essentially, line polarity reversal is used to signal the receiving telephone that a call is coming in. In other countries around the world polarity reversals are used to signal a variety of telephony events.
Another application of polarity reversal involves what is known as caller identification or caller ID. In a caller ID system, the telephone number of the calling party is transmitted to the called telephone and displayed on a screen prior to the called telephone being answered. A polarity reversal can thus provide an indication that caller ID information is forth coming.
Conventional implementations of on-hook polarity detection use current bleed resistors or opto-couplers to sense the polarity. The bleed resistor solution decreases the input impedance (generally undesirable) and is also subject to noise problems. The opto-coupler solution involves an E-O-E conversion; i.e., comprises a first converter that transforms an electrical current into a light signal, and a second converter that changes the light signal back into an electrical current. Such an electric-optic-electric conversion process draws significant current to turn on. While this is tolerable in certain countries where impedance limitation requirements have been relaxed specifically for this type of circuitry, these circuits cannot meet the requirements of more stringent countries. Accordingly, using the existing state of the art methods, one cannot build a universal telephony interface polarity detector, which is capable of meeting the requirements of all countries, whether the current bleed resistor or the opto-coupler system is employed.
In view of the widespread use of polarity modulation (i.e., the varying of the polarity of a voltage across the tip and ring inputs to a telephone line) to transmit data to telephone receiving equipment, a need exists in the art for an improved technique of accurately detecting the polarity of a voltage, and more importantly, a shift in said polarity, across telephone equipment. Preferably, such an improved technique would allow an input impedance that is high enough to satisfy the impedance limitation requirements of all countries, including those with the most stringent standards. Such a technique would also preferably be impervious to noise problems, and operable down to a relatively low voltage.